Lead-free frit

ABSTRACT

A lead-free frit suitable for use on aluminum, consisting essentially of: 
     
         ______________________________________                                    
 
    
     33-42 wt. %            SiO 2                                           
8-13                   TiO 2                                           
0.5-2.5                SnO 2                                           
0.5-2.5                BaO                                                
2.5-5.0 in the                                                            
absence of Ce 2  O 3  ;                                             
0-4.5 if Ce 2  O 3  is                                              
present.               SrO                                                
2.2-4.3                ZnO                                                
13-26                  Na 2  O                                         
4-9                    K 2  O                                          
0.7-6.0                Li 2  O                                         
0.5-4.5                B 2  O 3                                     
1.0-3.5                Sb 2  O 3                                    
0-0.8                  Ce 2  O 3                                    
0.5-5.0                P 2  O 5                                     
0.2-1.3                MoO 3                                           
______________________________________                                    
 
     The frit has a fusion temperature below about 1050° F. and displays good qualities of gloss, acid resistance, and adhesion.

FIELD OF THE INVENTION

This invention relates to an enamel frit which is especially useful forapplication to cooking and eating utensils made of aluminum or aluminumbased alloys.

BACKGROUND OF THE INVENTION

Frits which are used to form enamel coatings on underlying surfaces ofiron or steel usually have fusing temperatures which are above about1460° F. Those frits generally cannot be used on aluminum, which has amelting temperature of about 1217° F., well below that of such frits.(The common aluminum alloys melt at temperatures that are slightlyhigher than unalloyed aluminum but still substantially below the meltingpoint of such frits.) In order to be fusible on a surface utensil ofaluminum or its alloys, a frit must, among other criteria, have a fusingtemperature no higher than about 1050° F. In order to formulate such afrit, lead oxide was once included as a component of the frit; it actedas a flux and lowered the fusing temperature. However, it is now wellknown that even very small quantities of lead (or lead oxide) in a fritcan be toxic, where the frit is on a utensil which comes into contactwith food. In use of the utensil the lead oxide will gradually bedissolved from the enamel and may appear in food or liquid heated orserved in the utensil. It therefore has become desirable to avoid theuse of all lead oxide in frits to be applied to eating or coolingutensils.

One substitute for lead oxide, as a means of reducing the fusing pointof a frit, was bismuth oxide. Bismuth is not itself toxic; however, itwas found that virtually all large volume, low cost, sources of bismuthoxide contained small proportions of lead oxide as a trace impuritywhich was carried over into the frit. Even if lead oxide is only presentin an amount greater than 5 ppm, it is potentially harmful. Thus it hasnot been practical to provide a frit suitable for use on aluminum byusing bismuth oxide; effective amounts of it tend to carry more thanabout 5 ppm lead into the enamel.

The elimination of lead and bismuth oxides as fluxes for producing lowmelting frits has made it difficult to discover a frit which has anadequate overall combination of the necessary properties. In addition tohaving a fusing temperature below about 1050° F., a frit for use onaluminum must also have adequate chemical durabililty, which isgenerally characterized as acid resistance. Further, it must have goodadherence to the underlying base, whether a casting, extrusion or sheetaluminum; it must have good spall resistance; it must be capable ofaccepting coloring agents and pigments to yield a range of opaque colorsfrom very dark to pastels; and after firing it must display areflectance and a specular gloss which are adequate to meet aestheticrequirements.

It has proven unusually difficult to discover a frit which meets theserequirements while at the same time avoiding the use of essentially alllead and other toxic heavy metal oxides.

THE PRIOR ART

Fraser et al U.S. Pat. No. 2,660,531 shows a lead-free, fluorine-freefrit for aluminum, but contains high proportions of RO oxides andaluminum oxide. Kadish et al U.S. Pat. No. 2,827,393 teaches a lead-freeenamel for use on aluminum, but requires high BaO, B₂ O₃, and low SiO₂.

Commons et al U.S. Pat. No. 2,890,964 requires fluorine and zirconia,and utilizes high B₂ O₃ and TiO₂. Judd U.S. Pat. No. 2,925,351 shows alead-free frit for aluminum which has a very low content of RO oxidesand high Sb₂ O₅ content. Van Dolah et al U.S. Pat. No. 3,017,279 focuseson the use of cupric oxide and gold for color stability, in a wide rangeof frits.

Stradely U.S. Pat. No. 3,383,225 is directed to an acid resistant fritfor aluminum which contains no MoO₃ or SrO.

Little et al U.S. Pat. Nos. 3,458,344 and 3,463,646 teach frits for useon steel, and which require high P₂ O₅ B₂ O₃, and a low ratio betweentheir proportions.

Theisen U.S. Pat. No. 3,927,243 is specifically directed to a no-leadaluminum enamel which utilizes bismuth oxide in combination with TiO₂,and indicates that the frit should not contain ZnO, BaO, or SrO.

Knapp U.S. Pat. No. 4,256,497 shows a lead-free frit which is used forcoating alumina rather than aluminum, and which is high melting. NigrinU.S. Pat. No. 4,282,035 shows a frit which contains high SiO₂, ZrO₂, A1₂O₃, and relatively low alkali. Another Nigrin patent, No. 4,285,731,shows a lead-free frit for glass and glass ceramics, which uses high B₂O₃, BaO, and ZrO₂.

Eppler U.S. Pat. No. 4,312,951 is directed to a low melting lead-freeceramic which requires both alumina and fluorine.

Hommel et al U.S. Pat. No. 4,316,963 focuses on a leadless glazecomposition which is made with a molybdenate, but which also requiresalumina.

No. 4,361,654 teaches the use of MoO₃, and requires relatively high B₂O₃, and has a high fusion point as a result of the inclusion of severalhigh melting metal oxides.

No. 4,469,798 concerns a no-lead, low melting frit which uses high B₂ O₃fluorine, ZrO₂, and low soda. Francel U.S. Pat. No. 4,554,258 teaches alead-free frit with a very high bismuth content.

Nigrin patent No. 4,590,171 teaches a frit which is especially suitablefor contact with foods and food service applications but differs fromthe present frit in calling for low soda, more BaO, much more B₂ O₃,alumina, higher silica, zirconia, and fluorine.

West German patent document No. 1,091,831, teaches a lead-free aluminumfrit, but the frit has poor acid resistance.

BRIEF DESCRIPTION OF THE INVENTION

In accordance with this invention a frit is provided which, on an oxidebasis as melted, is comprised essentially of the following components:

    ______________________________________                                                   Broad Range, Preferred                                             RO.sub.2   Wt. %        Composition                                           ______________________________________                                        SiO.sub.2  33-42        37.0                                                  TiO.sub.2  8-13         12.2                                                  SnO.sub.2  0.5-2.5      1.4                                                   RO                                                                            BaO        0.5-2.5      1.0                                                   SrO        2.5-5.0 in the                                                                             3.0                                                              absence of Ce.sub.2 O.sub.3 ;                                                 0-4.5 if Ce.sub. O.sub.3 is                                                   present.                                                           ZnO        2.2-4.3      3.4                                                   R.sub.2 O                                                                     Na.sub.2 O 13-26        22.0                                                  K.sub.2 O  4-9          6.8                                                   Li.sub.2 O 0.7-6.0      4.3                                                   R.sub.2 O.sub.3                                                               B.sub.2 O.sub.3                                                                          0.5-4.5      1.9                                                   Sb.sub.2 O.sub.3                                                                         1.0-3.5      2.3                                                   Ce.sub.2 O.sub.3                                                                         0-0.8        0                                                     P.sub.2 O.sub.5                                                                          0.5-5.0      4.3                                                   MoO.sub.3  0.2-1.0      0.5                                                                           100.1                                                 ______________________________________                                    

In addition, Fe₂ O₃ or Cr₂ O₃ may be present as an impurity, or may beadded for coloring purposes in smallamounts:______________________________________ Fe₂ O₃ 0-0.3 Cr₂ O₃0-0.2______________________________________

Essentially no PbO or CdO should be present (as used in reference tothese components, "essentially no" means less than about 5 ppm). A1₂ O₃,F₂, and ZrO₂ are also undesirable and should not be added, although suchsmall proportions of them are not seriously harmful.

The total R₂ O preferably should not exceed 35% of the total, and thetotal RO₂ should be at least 42%.

Experimental testing has established the importance of the approximateranges recited for the individual oxides. The desired properties areobtained as a balance in a series of compromises that affect fusingpoint, acid resistance, fluidity, gloss, texture or smoothness, opacityand color. A component which improves one property (e.g., which lowersmelting point) often adversely affects another. For example, we havefound that adding Na₂ O lowers the melting point but hurts acidresistance, whereas adding silica improves acid resistance but raisesthe melting point. TiO₂ raises the melting point but lowers the gloss.SnO₂ improves opacity, but makes the frit more refractory. BaO reducesthe melting point but hurts acid resistance, texture, and smoothness.SrO increases the fluidity of the melted frit, but raises its fusionpoint; ZnO also increases fluidity and raises the melting point;moreover, it tends to make the frit refractory and reduces its acidresistance. Changes in K₂ O content have affects similar to those ofchanges in Na₂ O, but it is more expensive. Li₂ O lowers the meltingpoint and does not hurt acid resistance as much as potassium or soda,but is more expensive still. B₂ O₃ lowers the melting point, but isdetrimental to acid resistance and also lowers viscosity. Sb₂ O₃functions as an opacifier; reducing the amount of it lowers the fusingpoint. Ce₂ O₃ is optional but can be present; apparently it acts as aflux and it can to some extent reduce the needed SrO content. Theaddition of Ce₂ O₃ lowers the gloss slightly and makes the material morerefractory. P₂ O₅ acts as a glass former; increasing the amount of ittends to impair the acid resistance and decrease the viscosity of theglass. MoO₃ is required in small but important amount; it acts as anadhesion promoter and a flux.

Apart from the ingredients noted above, certain ingredients areimportant by their absence. PbO and CdO are undesirable in amounts morethan about 5 ppm, because of their toxicity. The flux should contain nosignificant amount of aluminum or zirconia, both of which are refractoryand raise the fusing point. Any significant amount of fluorine isunacceptable in the frit formulation because it tends to react with theunderlying aluminum metal and cause blistering and spalling.

DETAILED DESCRIPTION

Table I illustrates a number of examples of frits in accordance with theinvention. (In the following tables, each frit is expressed in terms ofthe percentages by weight of its oxide components, as melted.)

                  TABLE I                                                         ______________________________________                                        FRIT COMPOSITIONS                                                             Oxide,                                                                              Frit Nos.                                                               wt. % 1      2       3    4    5     6    7    8                              ______________________________________                                        SiO.sub.2                                                                           36.96  36.64   37.72                                                                              36.05                                                                              36.60 37.46                                                                              36.16                                                                              36.51                          TiO.sub.2                                                                           12.20  12.09   12.49                                                                              11.90                                                                              12.08 12.33                                                                              11.96                                                                              12.07                          SnO.sub.2                                                                           1.40   1.40    1.44 1.34 1.40  1.40 1.35 1.40                           BaO   1.01   1.01    1.04 0.97 1.01  1.01 0.98 1.01                           SrO   2.95   2.91    --   2.86 2.90  2.99 2.89 2.90                           ZnO   3.38   3.38    3.46 3.32 4.26  2.22 3.29 3.32                           Na.sub.2 O                                                                          22.00  21.83   22.52                                                                              21.45                                                                              21.81 22.27                                                                              21.52                                                                              21.76                          K.sub.2 O                                                                           6.77   6.74    6.90 6.58 6.73  6.86 8.84 6.65                           Li.sub.2 O                                                                          4.33   4.29    4.42 4.22 4.29  4.38 4.23 4.28                           B.sub.2 O.sub.3                                                                     1.88   1.88    1.94 4.36 1.88  1.88 1.82 1.84                           Sb.sub.2 O.sub.3                                                                    2.29   2.24    2.33 2.23 2.24  2.30 2.26 3.46                           Ce.sub.2 O.sub.3                                                                    --     0.79    0.79 --   --    --   --   --                             P.sub.2 O.sub.5                                                                     4.28   4.25    4.39 4.18 4.25  4.35  4.19                                                                              4.25                           MoO.sub.3                                                                           0.54   0.54    0.53 0.54 0.54  0.54 0.49 0.54                           Total                                                                         RO.sub.2                                                                            50.56  50.13   51.65                                                                              49.29                                                                              50.08 51.19                                                                              49.47                                                                              49.98                          Total                                                                         RO    7.34   7.30    4.50 7.15 8.17  6.22 7.16 7.23                           Total                                                                         R.sub.2 O                                                                           33.10  32.86   33.84                                                                              32.25                                                                              32.83 33.51                                                                              34.59                                                                              32.69                          ______________________________________                                    

Frit No. 1 is the presently preferred frit. As can be seen, it containsno lead, cadmium, bismuth, alumina, zirconia, or fluorine. The frit iscompounded and melted in accordance with conventional techniques fromthe standard raw materials. It has a fusion temperature of about 1000°F. The material is applied as a water slip.

An enamel coating formed from frit in accordance with Frit No. 1 has a"Class A" acid resistance, as measured according to ASTM Citric AcidSpot Test Procedure C282-67. (In this test the fired surface is exposedto 10% citric acid for 15 minutes at room temperature. The surfacechange is observed and evaluated according to the relative degree ofetching.) The frit was opaque white and its adhesion was excellent.(Adhesion is measured by testing the spall resistance of the enamel inaccordance with the Porcelain Enamel Institute Technical Bulletin T-51,"Antimony Trichloride Spall Test For Porcelain Enameled Aluminum." Thattest requires exposure of the enamel surface of a 1% solution ofantimony trichloride, for 20 hours.) The frit had a smooth surfacetexture and was glossy when fired at 1000° F., 970° F. (low temperaturefiring), or 1030° F. (high temperature firing). (Gloss is evaluated overa range of firing temperatures because the actual temperature ofconventional commercial furnaces often deviates slightly from the setpoint; moreover, relatively cold spots and hot spots normally exit insome areas of any given furnace. To be satisfactory for use on aluminuma frit must display satisfactory gloss over a firing range of about970-1030° F.) The other characteristics were evaluated in theconventional manner used for evaluation of porcelain enamel surfaces.

Frit No. 2 was basically similar to Frit No. 1 but containedapproximately 0.8% of cerium oxide. This gave slightly lower gloss whenfired at 970° F., and the acid resistance was somewhat lower, butoverall the frit was acceptable.

Frit No. 3 was basically similar to Frit No. 1, but contained 0.8% CeO₃and no SrO. This caused no apparent difference from the preferred frit.

Frit No. 4 was similar to No 1 but with approximately 2.5% more B₂ O₃.This slightly lowered the gloss at 970° F. and somewhat lowered acidresistance.

Frit No. 5 was basically similar to No. 1 but contained approximately 1%more zinc oxide. The additional zinc oxide somewhat impaired acidresistance.

Frit No. 6 was basically similar to Frit No. 1 but contained about 1.2%less zinc oxide than the former. This frit gave substantially the sameresults as No. 1.

Frit No. 7 was basically similar to No. 1 but contained about 2% more K₂O. The fired enamel did not visibly differ from that of Frit No. 1.

Frit No. 8 was like Frit No. 1 but contained 1.2% more antimony oxide.This reduced the gloss if fired at 970° F. but improved acid resistance.

Table II shows a number of frits which were outside the scope of theinvention by reason of giving unacceptably poor overall properties afterfiring.

                                      TABLE II                                    __________________________________________________________________________    Oxide,                                                                            No.                                                                       Wt. %                                                                             9  10  11 12 13 14  15 16 17 18                                           __________________________________________________________________________    SiO.sub.2                                                                         38.10                                                                            38.29                                                                             37.49                                                                            38.55                                                                            37.33                                                                            38.70                                                                             37.80                                                                            36.32                                                                            36.12                                                                            34.94                                        TiO.sub.2                                                                         12.60                                                                            12.63                                                                             12.39                                                                            12.79                                                                            12.29                                                                            12.79                                                                             12.51                                                                            11.95                                                                            11.99                                                                            14.22                                        SnO.sub.2                                                                         1.44                                                                             1.46                                                                              -- 1.48                                                                             1.40                                                                             1.46                                                                              1.46                                                                             1.35                                                                             1.38                                                                             1.40                                         ZrO.sub.2                                                                         -- --  -- -- -- --  -- -- -- --                                           BaO 1.04                                                                             1.06                                                                              1.01                                                                             1.07                                                                             1.01                                                                             1.06                                                                              1.01                                                                             0.98                                                                             1.00                                                                             1.01                                         SrO -- 2.96                                                                              2.91                                                                             3.08                                                                             2.98                                                                             3.07                                                                              3.03                                                                             2.89                                                                             2.91                                                                             2.95                                         ZnO 3.46                                                                             --  3.44                                                                             3.56                                                                             3.43                                                                             3.55                                                                              3.44                                                                             3.35                                                                             3.34                                                                             3.38                                         Na.sub.2 O                                                                        22.68                                                                            22.80                                                                             22.34                                                                            22.99                                                                            22.12                                                                            22.99                                                                             22.52                                                                            21.57                                                                            21.58                                                                            22.00                                        K.sub.2 O                                                                         6.97                                                                             7.02                                                                              6.89                                                                             7.09                                                                             6.80                                                                             7.08                                                                              6.94                                                                             6.63                                                                             6.64                                                                             6.77                                         Li.sub.2 O                                                                        4.46                                                                             4.48                                                                              4.40                                                                             -- 4.17                                                                             4.33                                                                              4.43                                                                             4.24                                                                             4.24                                                                             4.33                                         B.sub.2 O.sub.3                                                                   1.94                                                                             1.96                                                                              1.88                                                                             1.99                                                                             1.88                                                                             1.96                                                                              1.92                                                                             1.86                                                                             1.86                                                                             1.88                                         Sb.sub.2 O.sub.3                                                                  2.33                                                                             2.36                                                                              2.35                                                                             2.38                                                                             2.29                                                                             2.41                                                                              -- 2.20                                                                             2.21                                                                             2.29                                         Ce.sub.2 O.sub. 3                                                                 -- --  -- -- -- --  -- -- -- --                                           Al.sub.2 O.sub.3                                                                  -- --  -- -- -- --  -- 1.92                                                                             -- --                                           P.sub.2 O.sub.5                                                                   4.42                                                                             4.44                                                                              4.35                                                                             4.47                                                                             4.30                                                                             --  4.38                                                                             4.19                                                                             4.20                                                                             4.28                                         MoO.sub.3                                                                         0.53                                                                             0.54                                                                              0.54                                                                             0.55                                                                             -- 0.59                                                                              0.54                                                                             0.54                                                                             0.53                                                                             0.54                                         F.sub.2                                                                           -- --  -- -- -- --  -- -- 1.99                                                                             --                                           __________________________________________________________________________    Oxide                                                                             No.                                                                       Wt. %                                                                             19  20 21  22 23  24 25  26  27                                           __________________________________________________________________________    SiO.sub.2                                                                         36.30                                                                             37.71                                                                            36.61                                                                             35.24                                                                            36.34                                                                             39.99                                                                            35.17                                                                             37.16                                                                             36.51                                        TiO.sub.2                                                                         12.01                                                                             12.43                                                                            12.14                                                                             11.61                                                                            11.95                                                                             13.23                                                                            11.66                                                                             12.32                                                                             12.04                                        SnO.sub.2                                                                         3.07                                                                              1.40                                                                             1.40                                                                              1.34                                                                             1.34                                                                              1.50                                                                             1.34                                                                              1.40                                                                              1.39                                         ZrO.sub.2                                                                         --  -- --  -- --  -- 0.95                                                                              --  --                                           BaO 1.01                                                                              1.01                                                                             1.01                                                                              0.97                                                                             1.01                                                                              -- 2.87                                                                              0.93                                                                              0.92                                         SrO 2.90                                                                              2.99                                                                             2.90                                                                              2.82                                                                             2.90                                                                              -- 2.83                                                                              2.99                                                                              2.92                                         ZnO 3.32                                                                              3.44                                                                             3.38                                                                              3.20                                                                             3.32                                                                              -- 3.22                                                                              3.44                                                                              3.35                                         Na.sub.2 O                                                                        21.64                                                                             22.47                                                                            21.84                                                                             20.95                                                                            21.60                                                                             23.80                                                                            20.94                                                                             22.17                                                                             21.70                                        K.sub.2 O                                                                         6.65                                                                              6.89                                                                             6.73                                                                              6.46                                                                             6.65                                                                              7.31                                                                             6.45                                                                              6.81                                                                              6.69                                         Li.sub.2 O                                                                        4.26                                                                              4.41                                                                             4.29                                                                              4.12                                                                             4.25                                                                              4.68                                                                             4.12                                                                              4.36                                                                              4.27                                         B.sub.2 O.sub.3                                                                   1.84                                                                              -- 1.88                                                                              6.53                                                                             1.58                                                                              1.76                                                                             3.69                                                                              1.81                                                                              1.75                                         Sb.sub.2 O.sub.3                                                                  2.23                                                                              2.35                                                                             2.29                                                                              2.18                                                                             2.23                                                                              2.44                                                                             2.18                                                                              2.30                                                                              2.27                                         Ce.sub.2 O.sub.3                                                                  --  -- --  -- --  -- --  --  --                                           Al.sub.2 O.sub.3                                                                  --  -- --  -- --  -- --  --  --                                           P.sub.2 O.sub.5                                                                   4.21                                                                              4.37                                                                             4.25                                                                              4.09                                                                             6.28                                                                              4.70                                                                             4.08                                                                              4.31                                                                              4.21                                         MoO.sub.3                                                                         0.54                                                                              0.54                                                                             1.27                                                                              0.49                                                                             0.54                                                                              0.58                                                                             0.49                                                                              --  0.48                                         F.sub.2                                                                           --  -- --  -- --  -- --  --  1.47                                         __________________________________________________________________________

Frit No. 9 was similar to No. 1, but contained no SrO. The enamelappeared glossy but displayed a low reflectance.

Frit No. 10 was simliar to No. 1, but contained no ZnO. The frit had avery poor shelf life, tending to agglomerate when stored, and theresulting enamel when fired at 970° F. had a low gloss.

Frit No. 11 was similar to No. 1, but contained no tin oxide. Acidresistance was poor.

Frit No. 12 was similar to No. 1 but contained no lithium. Thisseriously hurt its acid resistance. Moreover, the enamel had no gloss;it did not fire out on the test plate at the temperatures used.

Frit No. 13 was similar to No. 1, but contained no molybdenum oxide.This reduced the acid resistance, moreover it reduced gloss when firedat 970°.

Frit No. 14 was similar to No. 1, but contained no P₂ O₅. Thisdrastically reduced the acid resistance, and it reduced gloss when firedat 970° F.

Frit No. 15 was similar to Frit No. 1, but it contained no antimonyoxide. This caused spalling at the edges of the samples, reduced glosswhen fired at 1030° F. ("high temperature firing"), and hurt the acidresistance.

Frit No. 16 was similar to No. 1, but contained about 1.9% of aluminumoxide. This seriously reduced the acid resistance and reduced gloss whenfired at 970° F.

Frit No. 17 was similar to No. 1, but it contained about 2% fluorine.The enamel formed by this frit spalled severely, had lower acidresistance, and lost gloss when fired at 1030° F.

Frit No. 18 was similar to No. 1 but it contained approximately 2.0%more TiO₂ and 2.0% less SiO₂. This hurt its acid resistance.

Frit No. 19 ws similar to No. 1, but contained approximately 1.7% moretin oxide. This reduced the gloss when fired at 970° F.

Frit No. 20 was similar to No. 1 but it contained no boron oxide. Thisreduced the gloss at low temperature firing (970° F.).

Frit No. 21 was similar to No. 1 but contained .73% more molybdenumoxide. This decreased the fluidity of the glass and hurt its acidresistance.

Frit No. 22 was like No. 1 but contained 4.65% more B₂ O₃. This frit hadlow acid resistance.

Frit No. 23 was similar to No. 1 but had increased P₂ O₅. The producthad poor gloss when fired at low temperature.

Frit No. 24 was similar to No. 1 but with no zinc oxide, no bariumoxide, and no strontium oxide. These changes raised the melting pointand hurt the gloss slightly at the low firing temperature (970° F.)

Frit No. 25 was like No. 1 but contained about 1.9% more BaO. Thefluidity of the glass was increased and the acid resistance wasadversely affected.

Frit No. 26 was like No. 1 but without the molybdenum oxide. It has poorlow temperature gloss and a poorer acid resistance.

Frit No. 27 was like No. 1 but contained about 1.5% fluorine. The glosswas lost at high firing temperatures (1030° F.), the acid resistance waspoorer, and severe spalling occurred.

While the frit described herein is especially suitable for use onutensils of aluminum or its alloys because of its low fusion temperatureand its freedom from toxic heavy metals, it is not restricted to suchuse. It is in fact quite suitable for use on higher melting metals,e.g., steel and cast iron, and is in general useful wherever it isdesirable to provide an opaque, durable, adherent, glossy, chemicallyresistant enamel while still avoiding toxic components.

Having described the invention, what is claimed is:
 1. A lead-free fritsuitable for use on aluminum and its alloys, said frit consistingessentially of about:

    ______________________________________                                        33-42 wt. %            SiO.sub.2                                              8-13                   TiO.sub.2                                              0.5-2.5                SnO.sub.2                                              0.5-2.5                BaO                                                    2.5-5.0 in the                                                                absence of Ce.sub.2 O.sub.3 ;                                                 0-4.5% if Ce.sub.2 O.sub.3 is                                                 present.               SrO                                                    2.2-4.3                ZnO                                                    13-26                  Na.sub.2 O                                             4-9                    K.sub.2 O                                              0.7-6.0                Li.sub.2 O                                             0.5-4.5                B.sub.2 O.sub.3                                        1.0-3.5                Sb.sub.2 O.sub.3                                       0-0.8                  Ce.sub.2 O.sub.3                                       0.5-5.0                P.sub.2 O.sub.5                                        0.2-1.0                MoO.sub.3                                              ______________________________________                                    

said frit having a fusion temperature below about 1050° F.
 2. The fritof claim 1, consisting essentially

    ______________________________________                                        37.0%                SiO.sub.2                                                12.2                 TiO.sub.2                                                1.4                  SnO.sub.2                                                1.0                  BaO                                                      3.0                  SrO                                                      3.4                  ZnO                                                      22.0                 Na.sub.2 O                                               6.8                  K.sub.2 O                                                4.3                  Li.sub.2 O                                               1.9                  B.sub.2 O.sub.3                                          2.3                  Sb.sub.2 O.sub.3                                         4.3                  P.sub.2 O.sub.5                                          0.5                  MoO.sub.3                                                ______________________________________                                    


3. The frit of claim 1 or 2, containing essentially no PbO, CdO, F₂, A1₂O₃, ZrO₂ or Bi₂ O₃.
 4. The frit of claim 3 containing essentially no Ce₂O₃.
 5. The frit of claim 1 or 2, further comprising, as impuritycomponents,

    ______________________________________                                        0-0.2               Cr.sub.2 O.sub.3 ; and                                    0-0.3               Fe.sub.2 O.sub.3.                                         ______________________________________                                    